Validating and computing stability limits of human-in-the-loop adaptive control systems

ABSTRACT

Systems and methods for implementing and/or validating a model reference adaptive control (MRAC) for human-in-the-loop control of a vehicle system. A first operator model is applied to a first feedback-loop-based MRAC scheme, wherein the first operator model is configured to adjust a control command provided as an input to the MRAC scheme based at least in part on an actual action of the vehicle system and a reference action for the vehicle system with a time-delay. A stability limit of a first operating parameter is determined for the MRAC scheme based on the application of the first operator model to the first feedback-loop-based MRAC scheme. The MRAC scheme is validated in response to determining that expected operating conditions of the first operating parameter are within the determined stability limit of the first operating parameter.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/427,882, filed Nov. 30, 2016, entitled “SYSTEMS AND METHODS FORCOMPUTING STABILITY LIMITS OF HUMAN-IN-THE-LOOP ADAPTIVE CONTROLARCHITECTURES,” the entire contents of which is incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention described herein was made in the performance of work undera NASA contract, and is subject to the provisions of Public Law 96-517(35 USC 202) in which the Contractor has elected to retain title.

BACKGROUND

The present invention relates to adaptive control systems (e.g., forcontrolling the operation of an automobile, an airplane, etc.) based onobserved performance feedback. In particular, certain embodiments of thepresent invention relate to adaptive control systems that are configuredto provide control of a vehicle system in parallel with a humanoperator.

SUMMARY

Achieving system stability and a level of desired system performance isone of the major challenges arising in control theory when dealing withuncertain dynamical systems. While fixed-gain robust control designapproaches can deal with such dynamical systems, the knowledge of systemuncertainty bounds is required and characterization of these bounds isnot trivial in general due to practical constraints such as extensiveand costly verification and validation procedures. On the other hand,adaptive control design approaches are important candidates foruncertain dynamical systems since they can effectively cope with theeffects of system uncertainties online and require less modelinginformation than fixed-gain robust control design approaches.

In various embodiments, the invention provides an adaptive controller;namely, a model reference adaptive controllers (MRAC), where thearchitecture includes a reference model, a parameter adjustmentmechanism, and a controller. In this setting, a desired closed-loopdynamical system behavior is captured by the reference model, where itsoutput (respectively, state) is compared with the output (respectively,state) of the uncertain dynamical system. This comparison yields asystem error signal, which is used to drive an online parameteradjustment mechanism. Then, the controller adapts feedback gains tominimize this error signal using the information received from theparameter adjustment mechanism. As a consequence under proper settings,the output (respectively, state) of the uncertain dynamical systembehaves as the output (respectively, state) of the reference modelasymptotically or approximately in time, and hence, guarantees systemstability and achieves a level of desired closed-loop dynamical systembehavior.

While MRAC offers mathematical and design tools to effectively cope withsystem uncertainties arising from ideal assumptions (e.g. linearization,model order reduction, exogenous disturbances, and degraded modes ofoperations), the capabilities of MRAC when interfaced with humanoperators can be however quite limited. Indeed, in certain applicationswhen humans are in the loop, the arising closed loop with MRAC canbecome unstable. As a matter of fact, such problems are not only limitedto MRAC-human interactions and have been reported to arise in varioushuman-in-the-loop control problems including, for example, pilot inducedoscillations. To address these issues, some control designs may beconfigured to provide adaptive control as well as smart-cue/smart-gainconcepts. On the other hand, an analytical framework aimed atunderstanding these phenomena and that can ultimately be used to driverigorous control design is currently lacking. These observationsmotivate this study where the main objective is to develop comprehensivemodels from a system-level perspective and analyze such models todevelop a strong understanding of the aforementioned stability limits,in particular within the framework of human-in-the-loop MRACarchitectures.

With the human-in-the-loop, one critical parameter added to the controlproblem that can be responsible for instabilities is the human reactiondelays. The presence of time delays is a source of instability, whichmust be carefully dealt with and explicitly addressed in any controldesign framework. Delay-induced instability phenomenon may occur innumerous applications including robotics, physics, cyber-physicalsystems, and operational psychology. For example, in physics literatureeffects of human decision making process and reaction delays are studiedto understand the arising car driving patterns, traffic flowcharacteristics, traffic jams, and stop-and-go waves.

In terms of mathematical modeling of human behavior, many studies focuson developing a representative transfer function of the human in aspecific task within a certain frequency band. Along these lines, wecite three key models; i) human driver models, ii) McRuer crossovermodel, and iii) Neal-Smith pilot model. Human driver models are proposedin the context of car driving, specifically in longitudinalcar-following tasks in a fixed lane. While these models vary dependingon the degree of their complexity, their simplest form is a pure timedelay representing the dead time between arrival of stimulus andreaction produced by the driver. McRuer's model was on the other handproposed to capture human pilot behavior, to further understand flightstability and human-vehicle integration. Among many of its variations,this model is essentially an integrator dynamics with a time lag tocapture human reaction delays and a gain modulated to maintain aspecific bandwidth. Similarly, the Neal-Smith pilot model, which isessentially a first order lead-lag type compensator with a gain and timelag, can be utilized to study the behavior of human pilots.

In light of the above discussions, it is of strong interest tounderstand the limitations of MRAC when coupled with human operators ina closed-loop setting. For this purpose, here MRAC is first incorporatedinto a general linear human model with reaction delays. Through use ofstability theory, this model is then studied to reveal and compute itsfundamental stability limit, and the parameter space of the model wheresuch limit is respected—hence MRAC-human combined model produces stabletrajectories. An illustrative numerical example of an adaptive flightcontrol application with a Neal-Smith pilot model is utilized next todemonstrate the effectiveness of developed approaches.

In various implementations, the invention provides a comprehensivecontrol theoretic modeling approach, where the dynamic interactionsbetween a general class of human models and MRAC framework can beinvestigated. In some implementations, this modeling approach focuses onunderstanding how an ideal MRAC would perform in conjunction with ahuman model including human reaction delays and how such delays couldpose strong limitations to the stabilization and performance of thearising closed-loop human-MRAC architecture. To this end, the examplesand discussion provided in this disclosure present various approachesand the pertaining theory with rigorous proofs guaranteeing stabilityindependent of delays and conditions under which stability can be lost.These results pave the way toward studying more complex human modelswith MRAC, advancing the design of MRAC to better accommodate humandynamics, and driving experimental studies with an analyticalfoundation.

In one embodiment, the invention provides a method of implementing amodel reference adaptive control (MRAC) for a vehicle system. A firstoperator model is applied to a first feedback-loop-based MRAC scheme,wherein the first operator model is configured to adjust a controlcommand provided as an input to the MRAC scheme based at least in parton an actual action of the vehicle system and a reference action for thevehicle system with a time-delay. A stability limit of a first operatingparameter is determined for the MRAC scheme based on the application ofthe first operator model to the first feedback-loop-based MRAC scheme.The MRAC scheme is validated in response to determining that expectedoperating conditions of the first operating parameter are within thedetermined stability limit of the first operating parameter.

In some implementations, the first operating parameter is a time-delayparameter indicative of a delay between the occurrence of an actualaction and a corresponding corrective action applied by the operator toa user control. In some such implementations, the expected operatingparameters are determined to be within the determined stability limit ofthe first operating parameter in response to determining that the MRACscheme will cause the system in response to determining that the MRACwill ensure that operation of the vehicle system will remain stableregardless of the value of the time-delay parameter (i.e.,time-delay-independent stability). In other implementations, a range oftime-delay values is determined for which the feedback-loop-based MRACscheme will ensure that operation of the vehicle system remains stableand the MRAC is validated if a range of expected time-delay values for aparticular operator, a particular vehicle system, or for all operatorsis within the determined range of stable time-delay values.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a control system for a vehicle according toone embodiment.

FIG. 2 is a schematic diagram of a method for controlling an operationof a vehicle using a model reference adaptive control (MRAC) in thesystem of FIG. 1.

FIG. 3 is a schematic diagram of a method for controlling an operationof a vehicle in order to validate a MRAC in the system of FIG. 1.

FIG. 4 is a schematic diagram of a method for validating an MRAC for usein the system of FIG. 1 using modeling.

FIG. 5 is a graph of the location of the right most pole (RMP) of acharacteristic equation representative of a human-in-the-loop MRAC withrespect to a control penalty variable p for different pilot reactiontimes.

FIG. 6 is a graph of tracking and control signal curves for twodifferent values of the pilot reaction times in the example of FIG. 5.

FIG. 7 is a graph of the location of the right-most pole (RMP) of thecharacteristic equation representative of the human-in-the-loop MRACwith respect to a control penalty variable μ for different pilottransfer function pole locations.

FIG. 8 is a graph of tracking and control signal curves for twodifferent values of pilot transfer function pole locations.

FIG. 9 is a graph of the location of the right-most pole (RMP) of thecharacteristic equation representative of the human-in-the-loop MRACwith respect to a control penalty variable μ for different pilottransfer function zero locations.

FIG. 10 is a graph of tracking and control signal curves for twodifferent values of pilot transfer function zero locations.

FIG. 11 is a graph of the location of the right-most pole (RMP) of thecharacteristic equation representative of the human-in-the-loop MRACwith respect to a control penalty variable μ for different pilottransfer function gain values.

FIG. 12 is a graph of tracking and control signal curves for twodifferent values of the pilot transfer function gain.

FIG. 13 is a flowchart of a method for validating the stability of anMRAC for use in the system of FIGS. 1 and 2.

FIG. 14 is a flowchart of a method for tuning a controller based onlydetermined stability limits of an MRAC for use in the system of FIGS. 1and 2.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways.

FIG. 1 illustrates an example of a human-in-the-loop, feedback-basedcontrol system. A controller 101 includes an electronic processor 103that is communicatively coupled to a computer-readable, non-transitorymemory 105. The memory 105 stores instructions that, when executed bythe electronic processor 103, causes the controller to provide variousfunctionality of the system including certain functionality as describedherein. The controller 101 is communicatively coupled to one or moreactuators and/or vehicle systems 107 and is configured to provide acontrol signal to the actuator(s)/vehicle system 107 to dictate theoperation of the actuator(s)/vehicle system 107. The controller 101 isalso communicatively coupled to one or more sensors 109 that monitor oneor more performance variables/states of the system being controlled bythe system of FIG. 1. The controller 101 is also configured to receive auser control input from one or more user control(s) 111.

For example, the system illustrated in FIG. 1 may be implemented in anautomobile where the user control 111 includes an steering wheel of theautomobile, the actuator(s)/vehicle system 107 includes a steeringand/or braking system, and the sensor 109 includes a yaw sensorconfigured to monitor turning characteristics of the automobile. Inanother example, the system may be configured to control the steering ofan airplane. In still other examples, the system may be configured tocontrol other aspects and/or of other systems.

FIG. 2 illustrates an example of a method implemented by the controller101 for controlling the operation of the vehicle using a model referenceadaptive controller (MRAC) mechanism. The controller 101 receives acontrol command from the user control 111 indicative of a desired taskto be performed (e.g., adjusting the steering of an airplane or anautomobile). The controller 101 applies the control command as an inputto a reference model 201 to determine a target behavior or action to beperformed in response to the control command from the user control 111.The controller 101 also applies the control command as an input to avehicle system control 203 that determines an appropriate actuatorcommand, which is then transmitted as a control signal from thecontroller 101 to the actuator 107. The controller 101 then monitors anoutput from a sensor 109 to determine how the system responded to theactuator command. The actual performance of the system, as indicated bythe output of the sensor 109, is then compared to the expectedperformance as determined by the reference model 201. The differencebetween the actual performance and the expected performance isidentified as a system error. The controller 101 is configured to applya parameter adjustment 205 that subsequently adjusts the actuatorcommand provided by the vehicle system control 203 to the actuator 107.In this way, the controller 101 is configured to use feedback tominimize the system error and, as a consequence, the actual performanceapproaches the expected performance.

The control architecture illustrated in FIG. 2 enables the controller101 to adapt to changes in the actuator/system itself due to degradationand to adapt to external forces that influence the actual performance ofthe vehicle in such a way that the system continues to operate asexpected (i.e., actual performance matches/approaches expectedperformance). For example, in an implementation where the system ofFIGS. 1 and 2 is provided to control the steering of an airplane, thecontrol command from the user control 111 may indicate that the operatorintends to continue to fly straight. However, turbulence or a strongwind may cause the airplane to diverge from its intended course. As aresult, the expected performance (i.e., straight travel) will notmatch/equal the actual performance. Using the MRAC control architectureof FIG. 2, the controller 101 uses feedback from the sensor 109 toadjust the actuator command 107 that is provided to the actuator 107and, thereby, adapt to account for the strong wind or turbulence.

However, before the controller 101 is able to adjust the actuators 107in such a way that the actual performance is corrected to match theexpected performance, the pilot of the airplane may also notice that thepath of travel of the airplane is deviating from its intended straightpath. In response, the pilot may adjust the position of the user control111 in a way intended to offset/correct for the deviation in the path oftravel. Accordingly, the controller 101 and the human operator (via theuser control 111) both attempt to correct for the system error. However,the human-induced “correction” may inadvertently affect the ability ofthe controller 101 to correct the system error and, in some cases, theinterference of the human-induced correction and the MRAC implemented bythe controller 101 may, not only prevent the controller 101 fromcorrecting the system error, but may also cause the steering of theairplane to become unstable.

To study the effect of human interactions with the MRAC controlarchitecture, the system may be adjusted to apply an additional modeledfeedback loop mechanism. For example, a human dynamics model 301, asdiscussed in further detail below, may be provided as a control modeldesigned to represent an expected human response to detecting an actualperformance that does not match the expected performance. In this way,the control architecture provided by the system of FIG. 3 includes an“inner loop” 311, in which the MRAC is applied to correct for deviationsbetween actual performance and expected performance and also includes an“outer loop” 313 in which the control command provided to the vehiclesystem control 307 is adjusted based on an expected human response. Thehuman dynamics model 301 in the example of FIG. 3 can, in someimplementations be implemented on the same controller 101 that isactively applying the MRAC. In other implementations, the human dynamicsmodel 301 is simulated and/or represented mathematically on a separatecomputer system.

Furthermore, in still other examples, the performance capabilities ofthe MRAC can be evaluated through modeling instead of throughobservation of actual system performance. For example, we start with theblock diagram configuration given by FIG. 4. In FIG. 4, the outer loop413 architecture includes the reference that is fed into the humandynamics model 401 to generate a command for the inner loop 411architecture in response to the variations resulting from the uncertaindynamical system 407. In this setting, the reference input is what thehuman aims to achieve in a task (e.g., the “expected performance”), andthe uncertain dynamical system 407 represents the machine on which thistask is being performed. The inner loop 411 architecture includes theuncertain dynamical system 407 as well as the model reference adaptivecontroller components (i.e., the reference model 403, the parameteradjustment mechanism 409, and the vehicle system control 405).Specifically, at the outer loop 413 architecture, we consider a generalclass of linear human models with constant time-delay given by{dot over (ξ)}(t)=A _(h)ξ(t)+B _(h)θ(t−τ), ξ(0)=ξ₀  (1)c(t)=C _(h)ξ(t)+D _(h)θ(t−τ)  (2)where ξ(t)∈

^(n) ^(ξ) is the internal human state vector, τ∈

₊ is the internal human time-delay, A_(h)∈

^(n) ^(ξ) ^(×n) ^(ξ) , B_(h)∈

^(n) ^(ξ) ^(×n) ^(r) , C^(h)∈

^(n) ^(c) ^(×n) ^(ξ) , D_(h)∈

^(n) ^(c) ^(×n) ^(r) , c(t)∈

^(n) ^(c) is the command produced by the human, which is the input tothe inner loop architecture as shown in FIG. 4. Here, input to the humandynamics is given byθ(t)

r(t)−E _(h)×(t)  (3)where θ(t)∈

^(n) ^(r) , with r(t)∈

^(n) ^(r) being the bounded reference. Here x(t)∈

^(n) is the state vector (further details below) and E_(h)∈

^(n) ^(r) ^(×n) selects the appropriate states to be compared with r(t).Note that the dynamics given by (1), (2), and (3) is general enough tocapture, for example, linear time-invariant human models with time-delayincluding Neal-Smith model and its extensions.

Next, at the inner loop architecture, we consider the uncertaindynamical system given by{dot over (x)} _(p)(t)=A _(p) x _(p)(t)+B _(p) Λu(t)+B _(p)γ_(p)(x_(p)(t)), x _(p)(0)=x _(p) ₀   (4)where x_(p)(t)∈

^(n) ^(p) is the accessible state vector, u(t)∈

^(m) is the control input, δ_(p):

^(n) ^(p) →

^(m) is an uncertainty, A_(p)∈

^(n) ^(p) ^(×n) ^(p) is a known system matrix, B_(p)∈

^(n) ^(p) ^(×m) is a known control input matrix, and Λ∈

₊ ^(m×m) ∩D^(m×m) is an unknown control effectiveness matrix.Furthermore, we assume that the pair(A_(p), B_(p)) is controllable andthe uncertainty is parameterized asδ_(p)(x _(p))=W _(p) ^(T)σ_(p)(x _(p)), x _(p)∈

^(n) ^(p)   (5)where W_(p)∈

^(s×m) is an unknown weight matrix and σ_(p) ^(n) ^(p) →

^(s) is a known basis function of the form σ_(p)(x_(p))=[σ_(p) ₁(x_(p)), σ_(p) ₂ (x_(p)), . . . , σ_(p) _(s) (x_(p))]^(T). Note for thecase where the basis function σ_(p)(x_(p)) is unknown, theparameterization in (5) can be relaxed without significantly changingthe results of this invention by consideringδ_(p)(x _(p))=W _(p) ^(T)σ_(p) ^(nn)(V _(p) ^(T) x _(p))+ε_(p) ^(nn)(x_(p)),x _(p) ∈D _(x) _(p)   (6)where W_(p)∈

^(s×m) and V_(p)∈

^(n) ^(p) ^(×s) are unknown weight matrices, σ_(p) ^(nn): D_(x) _(p) →

^(s) is a known basis composed of neural networks functionapproximators, ε_(p) ^(nn): D_(x) _(p) →

^(m) is an unknown residual error, and D_(x) _(p) is a compact subset of

^(n) ^(p) .

To address command following at the inner loop architecture, letx_(c)(t)∈

^(n) ^(c) be the integrator state satisfying{dot over (x)} _(c)(t)=E _(p) x _(p)(t)−c(t), x _(c)(0)=x _(c) ₀   (7)where E_(p)∈

^(n) ^(c) ^(×n) ^(p) allows to choose a subset of x_(p)(t) to befollowed by c(t). Now, (4) can be augmented with (7) as{dot over (x)}(t)=Ax(t)+BΛu(t)+BW _(p) ^(T)σ_(p)(x _(p)(t))+B _(r) C(t),x(0)=x ₀  (8)where

$\begin{matrix}{A\overset{\bigtriangleup}{=}{\begin{bmatrix}A_{p} & 0_{n_{p} \times n_{c}} \\E_{p} & 0_{n_{c} \times n_{c}}\end{bmatrix} \in {\mathbb{R}}^{n \times n}}} & (9) \\{B\overset{\bigtriangleup}{=}{\left\lbrack {B_{p}^{T},0_{n_{c} \times m}^{T}} \right\rbrack^{T} \in {\mathbb{R}}^{n \times m}}} & (10) \\{B_{r}\overset{\bigtriangleup}{=}{\left\lbrack {0_{n_{p} \times n_{c}}^{T},{- I_{n_{c} \times n_{c}}}} \right\rbrack^{T} \in {\mathbb{R}}^{n \times n_{c}}}} & (11)\end{matrix}$and x(t)

[x_(p) ^(T)(t),x_(c) ^(T)(t)]^(T)∈

^(n) is the augmented state vector, x₀

[x_(p) ₀ ^(T),x_(c) ₀ ^(T)]^(T)∈

^(n), and n=n_(p)−n_(c). In this inner loop architecture setting, it ispractically reasonable to set E_(h)=[E_(h) _(p) ,0_(n) _(r) _(×n) _(c)], E_(h) _(p) ∈

^(n) ^(r) ^(×n) ^(p) , in (3) without loss of theoretical generalitysince a subset of the accessible state vector is usually availableand/or sensed by the human at the outer loop (but not the states of theintegrator).

Finally, consider the feedback control law at the inner looparchitecture given byu(t)=u _(n)(t)+u _(a)(t)  (12)where u_(n)(t)∈

^(m) and u_(a)(t)∈

^(m) are the nominal and adaptive control laws, respectively.Furthermore, let the nominal control law beu _(n)(t)=−Kx(t)  (13)with K∈

^(m×n), such that A_(r)

A−BK is Hurwitz. For instance, such K exists if and only if (A,B) is acontrollable pair. Using (12) and (13) in (8) next yields{dot over (x)}(t)=A _(r) x(t)+B _(r) c(t)+BΛ[u _(a)(t)+W^(T)σ(x(t))]  (14)where W^(T)

[Λ⁻¹W_(p) ^(T),(Λ⁻¹−I_(m×m))K]∈

^((s+n)×m) is an unknown aggregated weight matrix and σ^(T)(x(t))

[σ_(p) ^(T)(x_(p)(t)),x^(T)(t)]∈

^(s+n) is a known aggregated basis function. Considering (14), let theadaptive control law beu _(a)(t)=−Ŵ ^(T)(t)σ(x(t))  (15)where Ŵ(t)∈

^((s+n)×m) is the estimate of W satisfying the parameter adjustmentmechanism{dot over (Ŵ)}(t)=γσ(x(t))e ^(T)(t)PB, Ŵ(0)=Ŵ ₀  (16)where γ∈

₊ is the learning rate, and system error reads,e(t)

x(t)−x _(r)(t)  (17)with x_(r)(t)∈

^(n) being the reference state vector satisfying the reference system{dot over (x)} _(r)(t)=A _(r) x _(r)(t)+B _(r) c(t),x _(r)(0)=x _(r) ₀  (18)and P∈

₊ ^(n×n)∩S^(n×n) is a solution of the Lyapunov equation0=A _(r) ^(T) P+PA _(r) +R  (19)with R∈

₊ ^(n×n)∩S^(n×n). Since A_(r) is Hurwitz, it follows that there exists aunique P∈

^(n×n)∩S^(n×n) satisfying (19) for a given R∈

₊ ^(n×n)∩S^(n×n). Although we consider a specific yet widely studiedparameter adjustment mechanism given by (16), one can also considerother types of parameter adjustment mechanisms without changing theessence of this invention.

Based on the given problem formulation, the next section analyzes thestability of the coupled inner and outer loop architectures depicted inFIG. 4 in order to establish a fundamental stability limit forguaranteeing the closed-loop system stability (when this limit issatisfied by the given human model at the outer loop and the givenadaptive controller at the inner loop).

Fundamental Stability Limit Calculation

To analyze the stability of the coupled inner and outer looparchitectures introduced in the previous section, we first write thesystem error dynamics using (14), (15), and (18) asė(t)=A _(r) e(t)−BΛ{tilde over (W)} ^(T) T(t)σ(x(t)),e(0)=e ₀  (20)where{tilde over (W)}(t)

{circumflex over (W)}(t)−W∈

^((s+n)×m)  (21)is the weight error and e₀

x ₀−x_(r) ₀ . In addition, we write the weight error dynamics using (16)as{dot over ({tilde over (W)})}(t)=γσ(x(t))e ^(T)(t)PB, {tilde over(W)}(0)={tilde over (W)} ₀  (22)where {tilde over (W)}₀

Ŵ(0)−W. The following lemma is now immediate.

Lemma 1.

Consider the uncertain dynamical system given by (4) subject to (5), thereference model given by (18), and the feedback control law given by(12), (13), (15), and (16). Then, the solution (e(t), {tilde over(W)}(t)) is Lyapunov stable for all (e₀, {tilde over (W)}₀)∈

^(n)×

^((s+n)×m) and t∈

₊.

Proof.

To show Lyapunov stability of the solution (e(t), {tilde over (W)}(t))given by (20) and (22) for all (e₀, {tilde over (W)}₀)∈

^(n)×

^((s+n)×m) and t∈

₊, consider the Lyapunov function candidateV(e,{tilde over (W)})=e ^(T) Pe+γ ⁻¹ tr({tilde over (W)}Λ^(1/2))^(T)({tilde over (W)}Λ ^(1/2))  (23)

Note that V(0,0)=0,V(e,{tilde over (W)})>0 for all (e,{tilde over(W)})≠(0,0), and V(e,{tilde over (W)}) is radially unbounded.Differentiating (23) along the trajectories of (20) and (22) yields{dot over (V)}(e(t),{tilde over (W)}(t))=−e ^(T)(t)Re(t)≤0  (24)where the result is now immediate.

Since the solution (e(t),{tilde over (W)}(t)) is Lyapunov stable for all(e₀,{tilde over (W)}₀)∈

^(n)×

^((s+n)×m) and t∈

₊ from Lemma 1, this implies that e(t)∈L_(∞) and {tilde over(W)}(t)∈L_(∞). At this stage in our analysis, it should be noted thatone cannot use the Barbalat's lemma to conclude lim_(t→∞) e(t)=0. Toelucidate this point, one can write{umlaut over (V)}(e(t),{tilde over (W)}(t))=−2e ^(T)(t)R[A _(r)e(t)−BΛ{tilde over (W)} ^(T)(t)σ(e(t)+x _(r)(t))]  (25)where since x_(r)(t) can be unbounded due to the coupling between theinner and outer loop architectures, one cannot conclude the boundednessof (25), which is necessary for utilizing the Barbalat's lemma in (24).Motivated from this standpoint, we next provide the conditions to ensurethe boundedness of the reference model states x_(r)(t), which alsoreveal the fundamental stability limit (FSL) for guaranteeing theclosed-loop system stability. It is noted that two FSLs are providedbelow; namely, a delay-independent FSL and a delay-dependent FSL.

Delay-Independent FSL

A linear time invariant system subject to time delay can in some casesbe stable regardless of how large the time delay τ is. We present themathematical conditions under which the system at hand can bedelay-independent stable. For this, start with using (2) in (18), andfirst write

$\begin{matrix}\begin{matrix}{{{{\overset{.}{x}}_{r}(t)} = {{A_{r}{x_{r}(t)}} + {B_{r}\left( {{C_{h}{\xi(t)}} + {D_{h}{\theta\left( {t - \tau} \right)}}} \right)}}},} \\{= {{A_{r}{x_{r}(t)}} - {B_{r}D_{h}E_{h}{x_{r}\left( {t - \tau} \right)}} + {B_{r}C_{h}{\xi(t)}} -}} \\{{B_{r}D_{h}E_{h}{e\left( {t - \tau} \right)}} + {B_{r}D_{h}{r\left( {t - \tau} \right)}}}\end{matrix} & (26)\end{matrix}$

Next, it follows from (1) that{dot over (ξ)}(t)=A _(h)ξ(t)−B _(h) E _(h) x _(r)(t−τ)−B _(h) E _(h)e(t−τ)+B _(r) r(t−τ)  (27)

Finally, by letting φ(t)

[x_(r) ^(T)(t),ξ^(T)(t)]^(T), and using (26) and (27), one can write{dot over (φ)}(t)=A ₀φ(t)+A _(τ)φ(t−τ)+φ(.), φ(0)=φ₀  (28)where

$\begin{matrix}{A_{0}\overset{\bigtriangleup}{=}{\begin{bmatrix}A_{r} & {B_{r}C_{h}} \\0_{n_{\xi} \times n} & A_{h}\end{bmatrix} \in {\mathbb{R}}^{{({n + n_{\xi}})} \times {({n + n_{\xi}})}}}} & (29) \\{A_{\tau}\overset{\bigtriangleup}{=}{\begin{bmatrix}{{- B_{r}}D_{h}E_{h}} & 0_{n \times n_{\xi}} \\{{- B_{h}}E_{h}} & 0_{n_{\xi} \times n_{\xi}}\end{bmatrix} \in {\mathbb{R}}^{{({n + n_{\xi}})} \times {({n + n_{\xi}})}}}} & (30) \\{{\varphi( \cdot )}\overset{\bigtriangleup}{=}{\begin{bmatrix}{{{- B_{r}}D_{h}E_{h}{e\left( {t - \tau} \right)}} + {B_{r}D_{h}{r\left( {t - \tau} \right)}}} \\{{{- B_{h}}E_{h}{e\left( {t - \tau} \right)}} + {B_{h}{r\left( {t - \tau} \right)}}}\end{bmatrix} \in {\mathbb{R}}^{n + n_{\xi}}}} & (31)\end{matrix}$

As a consequence of Lemma 1 and the boundedness of the reference r (t),one can conclude that φ(.)∈L_(∞). We now state the following lemma thatis necessary for the main result of this invention.

LEMMA 2. Let P∈

₊ ^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾∩S^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾ and S∈

₊ ^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾∩S^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾ such thatthe linear matrix inequality (LMI)

$\begin{matrix}{F\overset{\bigtriangleup}{=}{\begin{bmatrix}{{A_{0}^{T}P} + {PA}_{0} + S} & {PA}_{\tau} \\{A_{\tau}^{T}P} & {- S}\end{bmatrix} < 0}} & (32)\end{matrix}$holds. Then, φ(t) of the dynamical system given by (28) is bounded forany τ∈

₊ and for all φ(t)∈

^(n+n) ^(ξ) and τ∈

₊.

PROOF. Consider the Lyapunov-Krasovskii functional candidate given byV(φ)=φ^(T) Pφ+∫ _(−τ) ⁰φ^(T)(t+μ)dμ  (33)and, since φ(.)∈L_(∞), let φ*∈

₊ be such that ∥φ(.)∥₂≤φ*. Differentiating (33) along the trajectory of(28) yields{dot over (V)}(φ(t))≤η^(T)(t)Fη(t)+2λ_(max)(P)φ*|η(t)∥₂  (34)where η(t)

[φ^(T)(t),φ^(T)(t−τ)]^(T). Since (32) holds, let k∈

₊ be such that k

−λ_(min)(F). Now, it follows from (34) that{dot over (V)}(φ(t))≤−k∥η(t)∥₂(∥η(t)∥₂−2k ⁻¹λ_(max)(P)φ*)  (35)and hence, there exists a compact set R

({η(t)∈

^(2(n+n) ^(ξ) ⁾: ∥η(t)∥₂≤2k⁻¹λ_(max)(P)φ*} such that {dot over(V)}(φ(t))≤0 outside of this set, which proves the boundedness of (28)for any τ∈

₊ and for all φ(0)∈

^(n+n) ^(ξ) and τ∈

₊.

Lemma 2 establishes the boundedness of not only the reference modelstates, the dynamics of which are given by (18), but also the internalhuman dynamics given by (1), and hence, x_(r)(t)∈L_(∞) and ξ(t)∈L_(∞).

Theorem 1.

Consider the uncertain dynamical system given by (4) subject to (5), thereference model given by (18), the feedback control law given by (12),(13), (15), and (16), and the human dynamics given by (1), (2), and (3).Then, e(t)∈L_(∞) and {tilde over (W)}(t)∈L_(∞). If, in addition, thereexist P∈

₊ ^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾∩S^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾ and S∈

₊ ^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾∩S^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾ such thatthe LMI given by (32) holds, then x_(r)(t)∈L_(∞), ξ(t)∈L_(∞), andlim_(t→∞) e(t)=0.

Proof.

As a consequence of Lemma 1, recall that e(t)∈L_(∞) and {tilde over(W)}(t)∈L_(∞). In addition, note that φ(.)∈L_(∞) in (28). Next, if thereexist P∈

₊ ^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾∩S^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾ and S∈

₊ ^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾∩S^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾ such thatthe LMI given by (32) holds, recall from Lemma 2 that x_(r)(t)∈L_(∞) andξ(t)∈L_(∞). Finally, since e(t)∈L_(∞), x_(r)(t)∈L_(∞), and {tilde over(W)}(t)∈L_(∞) ensure the boundedness of (25), it now follows from theBarbalat's lemma that lim_(t→∞) e(t)=0.

For the boundedness of all closed-loop system signals and lim_(t→∞)e(t)=0, Theorem 1 requires the fundamental stability limit given by theLMI (32) to hold. Note that this fundamental stability limit can beequivalently written in an equality form as0=A ₀ ^(T) P+PA ₀ P+A _(τ) S ⁻¹ A _(τ) ^(T) P+S+Q  (36)where P∈

₊ ^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾∩S^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾, S∈

₊ ^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾∩S^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾, and Q∈

₊ ^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾∩S^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾ with A₀and A_(τ) respectively given by (29) and (30). Importantly, in addition,note that A₀ and A_(τ) do not depend on any unknown parameters and theyonly depend on the given set of human model and reference modelparameters. As a consequence, for a given human model of the form (1),(2), and (3), if the fundamental stability limit given by (36) (or,equivalently (32)) holds with respect to a judiciously chosen referencemodel parameters, then the closed-loop system trajectories areguaranteed to be stable.

Notice above that we have employed a time-domain technique based on aLyapunov-Krasovskii functional to prove delay independent stability. Alarge body of literature was devoted to this effort where one main focuswas to reduce the inherent conservatism imposed by the choice ofcandidate functionals. Another method would be to employ frequencydomain tools where one instead studies the eigenvalues of thecorresponding linear time invariant system with time delay. For example,consider the nominal part of (28); e.g., φ(.)=0, with τ→∞. In this case,the system will behave like an open loop system whose stability isdetermined by the eigenvalues of A₀. For the system to be stable in thissetting, A₀ must be Hurwitz, which also makes it invertible. Next, wenote that the characteristic function of the dynamical systemf:=det[sI−A ₀ −A _(τ) e ^(−sτ)]  (37)can be rearranged asdet[I−(sI−A ₀)⁻¹ A _(τ) e ^(−sτ)]*det[sI−A ₀]  (38)

Note that for the class of time-delay systems being considered here, asa parameter of interest; e.g., delay, changes, the system can switchfrom a stable to unstable regime (or vice versa) if and only if thesystem has imaginary eigenvalues s=jω. Investigation of whether or notsuch a switch could arise then requires studying the zeros of the systemcharacteristic function (38) at s=jω, where ω<0 without loss ofgenerality. On the imaginary axis however only the first determinant canbe zero since the second determinant is always non-zero owing to A₀being Hurwitz. Denoting with ρ(.) the spectral radius and noticing that|e^(−jωτ)|=1, we have the following theorem.

Theorem 2.

The dynamical system given by (28) with φ(.)=0 is asymptotically stableindependent of delay if and only if

i) A₀ is asymptotically stable;

ii) ρ((jωI−A₀)⁻¹A_(τ))<1, ∀ω>0; and

iii) either a) ρ(A₀ ⁻¹A_(τ))<1 or b) ρ(A₀ ⁻¹A_(τ))=1 anddet(A₀+A_(τ))≠0.

Implementing the steps in the above theorem are straightforward.Condition i) can be checked by a standard eigenvalue computation, whilecondition ii) requires sweeping of the frequency ω>0. Here one generatesthe matrix (jωI−A₀)⁻¹A_(τ) and for a given ω, computes the eigenvalues.If all these eigenvalues fall into the unit circle then condition ii) issatisfied for this ω. This process is repeated for all ω. Note that theinverse matrix operation here will guarantee that, for sufficientlylarge ω, condition ii) will always be satisfied as the spectral radiuswill keep shrinking. Checking of condition iii) is much simpler as itdoes not require parametric scanning but only computation ofeigenvalues. Note that condition iii) is the special case of conditionii) computed at ω=0.

Corollary 1.

Let the human dynamics given by (1), (2), and (3) be a single-inputsingle-output system (SISO) with gain k_(p). Then, for (28) with φ(.)=0to be delay-independent stable, it is necessary that

$\begin{matrix}{k_{p} < \frac{1}{\rho\left( {A_{r}^{- 1}B_{r}E_{h}} \right)}} & (39)\end{matrix}$holds.

Proof.

Start with (29) and (30) and rewrite the characteristic function (37)explicitly asf:=det[sI−A _(r) +B _(r)(C _(h)(sI−A _(h))⁻¹ B _(h) +D _(h))E _(h) e^(−τs)]  (40)which simplifies tof:=det[sI−A _(r) +B _(r) E _(h) G(s)e ^(−τs)]  (41)where G(s) is the scalar transfer function corresponding to the SISOsystem given by (1) and (2). Note that the above expression is in theexact form as (37); hence, for (28) with φ(.)=0 to be delay-independentstable, it is necessary that condition i) of Theorem 2 holds, which inthis case requires that A_(r) must be Hurwitz. As per the constructionin (13) this always holds. Then, invoking condition ii) in Theorem 2 atω=0, and recalling that k_(p)=G(0), we haveρ((−A _(r))⁻¹(B _(r) E _(h))G(0))<1  (42)which gives (39), and hence, the proof is now complete.

It is worthy to note that the results in Corollary 1 can be furtherimproved in many practical situations. For example, observe that thereference input to the human model and the human command are ofdimension one in the SISO case. In addition, since generally the outerloop and inner loop command following objectives are the same, note thatE_(h) _(p) =E_(p). Thus, in view of these, the following result is nowimmediate.

Corollary 2.

Given E_(h) _(p) =E_(p) and under the conditions in Corollary 1, thenecessary condition for the human-in-the loop MRAC model (28) withφ(.)=0 to be delay-independent stable is given byk _(p)<1  (43)

Proof.

Note that A_(r) ⁻¹B_(r) and E_(h) in (39) are column vectors. Therefore,we have ρ(A_(r) ⁻¹B_(r)E_(h))=|E_(h)A_(r) ⁻¹B_(r)|. Since in the scalarcase, E_(h)A_(r) ⁻¹B_(r)=−1, then (43) follows.

In the above corollary, we prove that the human gain must be less thanone such that (28) with φ(.)=0 can have a chance to be delay-independentstable. The sufficiency can be numerically checked by studying conditionii) of Theorem 2 (see the next section). What is interesting in theabove analysis is that human's aggressiveness as measured by k_(p) canbe a strong limiting factor that ruins delay-independent stability. Inthe case when MRAC deals with a highly aggressive human behavior withk_(p)>1, it is impossible to avoid instability for some delay values τ.Moreover, since by the design of stable MRAC we have zero steady-stateerror in tracking, the necessary condition k_(p)<1 is solely inherent tothe human's gain and holds irrespective of the controller gain K. Whilein many cases it is reasonable to assume that the human model can beconsidered as SISO dynamics; e.g., when the human produces a singleoutput to steer a manipulator, in the case when an auto-human model isutilized in multi-input multi-output (MIMO) form, the necessarycondition (42) can be revised as followsρ(A _(r) ⁻¹ B _(r) |G(0)|E _(h))<1  (44)where [G(0)] denotes the matrix transfer function of the MIMO auto-humanmodel with s=0 in its all entries.

It is important to note that while guaranteeing delay-independentstability in a dynamical system is attractive as this makes the systemcompletely immune to destabilizing effects of delays, in some cases bythe nature of the problem, delay-independent stability cannot bepossible as is the case above for k_(p)>1. Moreover, a trade-off indelay-independent stable cases is system's performance, which maydeteriorate for large delays although stability is preserved. In lightof this, we now turn our attention to the case when delay-independentstability is not possible, or not desired, and hence, system stabilityis affected by the numerical value of the delay in the dynamical system.

Delay Dependent FSL

Delay-independent FSL given in the previous section guarantees theboundedness of all closed loop system signals and lim_(t→∞) e(t)=0 forany τ∈

₊. Since the time delay in human dynamics can in general be known inpractice for certain applications, at least within a certain range, itis possible to relax these conditions by utilizing the delay informationin the stability analysis. Towards this goal, we first provide thefollowing lemma.

Lemma 3.

Consider the following system dynamics given byż(t)=Fz(t)+Gz(t−τ)+h(t,z(t)),z(0)=z ₀  (45)where z(t)∈

^(n) is the state vector, F∈

^(n×n) and G∈

^(n×n) are constant matrices, τ is the time delay, and h(t, z(t)) ispiecewise constant and bounded nonlinear forcing term, which is ingeneral a function of state z. If the homogeneous dynamical system givenbyż(t)=Fz(t)+Gz(t−τ)  (46)is asymptotically stable, then the states of the original inhomogeneousdynamical system given by (45) remains bounded for all times.

Proof.

Since h(t, z(t)) is piecewise continuous and bounded, this signal can beconsidered as an exogenous input to the system with the transferfunctionG(s)=(sI−(F+Ge ^(−τs)))⁻¹  (47)

Under the assumption that the homogeneous system (46) is asymptoticallystable, then we have that all of the infinitely many roots of thecharacteristic equationdet(sI−(F+Ge ^(−τs)))=0  (48)of the system (47), have strictly negative real parts. Therefore, theoutput z(t) of the dynamical system remains bounded.

Having established Lemma 3, we are now ready to state the second mainresult of this invention, which provides a more relaxed delay-dependentstability condition for the overall human-in-the-loop system andconvergence of the system error, e(t), to zero.

Theorem 3.

Consider the uncertain dynamical system given by (4) subject to (5), thereference model given by (18), the feedback control law given by (12),(13), (15), and (16), and the human dynamics given by (1), (2), and (3).Then, e(t)∈L_(∞) and {tilde over (W)}(t)∈L_(∞). If, in addition, thereal parts of all the infinitely many roots of the followingcharacteristic equationdet(sI−(A ₀ +A _(τ) e ^(−τs)))=0  (49)have strictly negative real parts, then x_(r)(t)∈L_(∞), ξ(t)∈L_(∞), andlim_(t→∞) e(t)=0.

Proof.

As a consequence of Lemma 1, recall that e(t)∈L_(∞) and {tilde over(W)}(t)∈L_(∞). In addition, note that φ(.)∈L_(∞) in (28). Therefore, ifall of the roots of the characteristic equation given by (49) havestrictly negative real parts, making the homogeneous equation{dot over (φ)}(t)=A ₀φ(t)+A _(τ)φ(t−τ)  (50)asymptotically stable, then, per Lemma 3, φ(t)

[x_(r) ^(T)(t),ξ^(T)(t)]^(T)∈L_(∞). Finally, since e(t)∈L_(∞),x_(r)(t)∈L_(∞), and {tilde over (W)}(t)∈L_(∞) ensure the boundedness of(25), it now follows from the Barbalat's lemma that lim_(t→∞) e(t)=0.

Note that there are several methods in the literature for the analysisof the root locations of (49). The four most-used methods are TRACE-DDE,DDE-BIFTOOL, QPMR, and Lambert-W function. In essence, one provides thematrices A₀ and A_(τ) as well as the delay τ to these methods, whichthen return the numerical values of the rightmost root locations of(49). In some sense, these methods perform a nontrivial approximationwith which they are able to identify the most relevant roots—therightmost roots. In the illustrative numerical example provided below,we employ TRACE-DDE readily available for download athttps://users.dimi.uniud.it/˜dimitri.breda/research/software/.

Illustrative Example

Consider the longitudinal motion of a Boeing 747 airplane linearized atan altitude of 40 kft and a velocity of 774 ft/sec with the dynamicsgiven by{dot over (x)}(t)=A _(p) x(t)+B _(p)(u(t)+W ^(T)σ(x(t))), x(0)=x ₀  (51)where x(t)=[x₁(t),x₂(t),x₃(t),x₄(t)]^(T) is the state vector. Note that(51) can be equivalently written as (4) with Λ=I. Here, x₁(t), x₂(t),and x₃(t) respectively represent the components of the velocity alongthe x, z and y axes of the aircraft with respect to the reference axes(in crad/sec), and x₄(t) represents the pitch Euler angle of theaircraft body axis with respect to the reference axes (in crad). Recallthat 0.01 radian=1 crad (centriradian). In addition, u(t)∈

represents the elevator control input (in crad). Finally, W∈

³ is an unknown weighting matrix and σ(x(t))=[1, x₁(t), x₂ (t)]^(T) is aknown basis function. In the following simulations, we set W=[0.1 0.3−0.3]. The dynamical system given in (51) is assumed to be controlledusing a model reference adaptive controller. In addition, the aircraftis assumed to be operated by a pilot whose Neal-Schmidt Model is givenby

$\begin{matrix}{k_{p}\frac{{T_{p}S} + 1}{{T_{z}S} + 1}e^{{- \tau}\; s}} & (52)\end{matrix}$Where k_(p) is the positive scalar pilot gain, T_(p) and T_(z) arepositive scalar time constants, and τ is the pilot reaction time delay.The values of the parameters used in the simulations are provided inTable 1.

To obtain the nominal controller K, a linear quadratic regulator (LQR)approach is utilized with the following objective function to beminimizedJ(.)=∫₀ ^(∞)(x ^(T)(t)Qx(t)+μu ²(t))dt  (53)where Q is a positive-definite weighting matrix of appropriate dimensionand μ is a positive weighting scalar. Notice that the frameworkdeveloped above is not limited to a particular design method for thenominal controller. To this end, this task can be handled by a number ofdifferent ways. Here LQR is utilized for convenience reasons. In thissetting, the selection of the weighing matrices, as expected, willaffect the resulting nominal controller gain K in (13), which in turnwill determine the reference model dynamics (18). In the followingsimulation studies, the effect of the weighting matrices, and thus theeffects of reference model parameters on system stability areinvestigated for various values of pilot model parameters. To facilitatethe analysis, reference model parameter variations is achieved mainly bymanipulating the control penalty variable μ.

TABLE 1 T_(p) 1 T_(z) 5 τ 0.5 A_(p) [−0.003 0.039 0 −0.322; −0.065−0.319 7.740 0; 0.020 −0.101 −0.429 0; 0 0 1 0] B_(p) [0.010; −0.1800;−1.160; 0] B_(p) [0.0100 −0.1800 −1.1600 0]^(T) E_(p) [0 0 0 1] E_(h) [00 0 1 0] B_(r) [0 0 0 0 1]^(T) Q diag([0 0 0 1 2.5])

Note that the purpose of the numerical examples provided in this sectionis to verify the theoretical stability predictions of the proposedframework. Therefore, the simulation results are created to present thestability/instability of the closed loop system without paying attentionto enhanced transient response characteristics.

Delay-Independent Stability: LMI Approach:

We set k_(p)=½ without loss of generality and investigate whether or notthe closed loop is delay-independent stable. Specifically, we first usethe LQR control designer in MATLAB with μ=1.0 to design K, which returnsK=[−0.0185, 0.0815, −1.5809, −2.7560, −1.5811]. Next the matrices A₀ andA_(τ) are constructed based on the information provided on Table 1.Assigning P and S as positive definite variables greater than 0.5I∈

^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾ while imposing the negativity constraintin (32) as F<−0.1I∈

^((n+n) ^(ξ) ^()×(n+n) ^(ξ) ⁾, the YALMIP LMI optimization toolboxreturns a feasible set of matrices P and S, indicating that theclosed-loop system is delay-independent stable.

Delay-Independent Stability: Frequency-Domain Approach:

To be consistent with the previous subsection, we set k_(p)=½ and μ=1.0in the LQR optimization. Based on Corollary 2, since k_(p)<1 and A_(r)is Hurwitz, the necessary conditions for delay-independent stability aresatisfied. Next, the sufficient conditions in Theorem 2 are to bechecked simply by computing the metric in condition ii)-iii) of thetheorem with respect to ω≥0. We find out that the metric value starts atk_(p)=½ when ω=0 (condition iii)) and decreases for larger ω≠0(condition ii)), remaining always less than 1. That is, the closed loopsystem will remain stable for any choice of delay τ. Keeping μ=1.0 butletting k_(p)=0.95 has only negligible effects on K, again with thesystem remaining delay independent stable under the conditions ofTheorem 2. On the other hand, selecting k_(p)=1.05 violates the theoremand the system loses its delay-independent stability characteristics.

Delay-Dependent Stability: Effect of Control Penalty on System Stabilityfor Different Pilot Reaction Time Delays:

To investigate the effects of the reference model parameter variationson the stability of the closed loop system, the control weight μ ismanipulated by assigning values in the range [0, 50]. Then, therightmost pole (RMP) of the system, whose characteristic equation isgiven by (49), is plotted against these μ values. This procedure isrepeated for various pilot reaction time delays and the results arepresented in FIG. 5.

FIG. 5 reveals several interesting results. First, it is shown that ifthe reference model dynamics is not chosen carefully with an appropriateμ value, then the human-in-the-loop adaptive control system can beindeed unstable. Second, it is seen that the closed loop system can bestable for small and large values of the parameter¹ and be unstable inbetween. Third, it is observed that as the pilot reaction time delayincreases, the unstable region of μ gets larger as indicated by RMP>0.

It is predicted in FIG. 5 that for μ=10, pilot reaction time delaysτ=0.2 and τ=0.5 results in a stable and unstable system, respectively.Time domain tracking and control signal plots presented in FIG. 6confirm this prediction. As noted earlier, the simulation results areemployed to verify the theoretical stability predictions of the proposedmethod and therefore controllers are not tuned to obtain the besttransient response. The investigation of the effect of thehuman-controller interactions on the transient response will beaddressed in future research.

Delay-Dependent Stability: Effect of Control Penalty on System Stabilityfor Different Values of Pilot Model Poles:

The poles of the pilot model (52) represent how fast the pilot respondsto changes in the aircraft pitch angle, which can also be interpreted aspilot aggressiveness. In this section, the effect of pilotaggressiveness on system stability is investigated while assigningvalues to the control penalty μ from 0 to 50.

FIG. 7 depicts the effect of the pilot pole locations on the RMP. Thezero location and the time-delay of the pilot model, are kept at theirnominal values of −1 and 0.5, respectively. It is seen from the figurethat, in general, stable-unstable-stable transition is observed forincreasing values of μ and, as expected, higher values of poles,corresponding to faster pilot response, decrease the μ region ofstability.

FIG. 8 depicts the tracking and control signal curves for two pilotmodel pole locations; that is, −0.175 and −0.2, when μ=10. As predictedin FIG. 7, the closed loop system remains stable when the pole islocated at −0.175 and becomes unstable when the pole is at −0.2.

Delay-Dependent Stability: Effect of Control Penalty on System Stabilityfrom Different Values of Pilot Model Zeros:

In this section, the effect of zeros of the pilot transfer function (52)on system stability is investigated when control penalty pi takes valuesin the range [0,50]. The pole location and the time delay of the pilottransfer function are kept at their nominal values of −0.2 and 0.5,respectively. Changes in the zero location of the model can beinterpreted as an adjustment to the “lead” nature of the pilot, which isrelated to pilot's anticipation capabilities.

As seen in FIG. 9, stable-unstable-stable transition structure stillexists, in general, for increasing μ values. Furthermore, it is seenthat when the pilot transfer function does not have a zero, a large μregion of instability arises. It is noted that for the given nominalvalues of the system parameters, no value of zero can make the systemalways stable, regardless of the μ value, since delay-independence isdetermined only by the pilot's gain k_(p).

FIG. 10 presents tracking and control signal curves for pilot model zerolocations −0.2 and −0.909, for the case when μ=1. As predicted in FIG.9, the closed loop system becomes stable for the former and unstable forlatter zero value.

Delay-Dependent Stability: Effect of Control Penalty on System Stabilityfor Different Values of Pilot Model Gains:

The pilot gain in kp in (52) determines the intensity of the responsethat the pilot gives to the pitch angle deviations in the aircraft. Insome sense, this gain also represents the aggressiveness of the pilot.

Stability properties of the pilot-in-the-loop system depending on thenominal control penalty μ and the pilot gain k_(p) is presented in FIG.11, where the RMP vs μ is plotted for certain values of k_(p). In theseanalyses, the pole and zero locations and time-delay of the pilottransfer function are kept at their nominal values of −0.2, −1, and 0.5,respectively. From the figure, stable-unstable-stable stabilitytransition is once again observed for increasing values of μ. On theother hand, it is seen that, similar to the trend for the pilot polelocation, as the pilot gain increases, the μ stability region shrinks.These results confirm the well-known adverse effects of high gain ofpilots on system stability, such as pilot-induced oscillations.

It is predicted in FIG. 11 that the closed loop system will be stablefor k_(p)=4 and unstable for k_(p)=5, when μ=10. This is confirmed bythe results presented in FIG. 12, where time domain tracking and controlsignal curves are plotted for these gain values.

To summarize, the presented invention analyzed human-in-the-loop modelreference adaptive control architectures and explicitly derivedfundamental stability limit for both delay-independent anddelay-dependent stability cases. Specifically, this stability limitresults from the coupling between outer and inner loop architectures,where the outer loop portion includes the human dynamics modeled as alinear dynamical system with time delay and the inner loop portionincludes the uncertain dynamical system, the reference model, theparameter adjustment mechanism, and the controller. We showed that whenthe given set of human model and reference model parameters satisfy thisstability limit, the closed-loop system trajectories are guaranteed tobe stable. The theoretical stability predictions of the proposedapproach were verified via several simulation studies presented above.While the main focus of this invention was to reveal and computestability limit of human-in-the-loop model reference adaptive controlarchitectures, the effect of the controller design parameters on thetransient response is also another important research direction thatwill be taken into consideration as a future research direction.

The techniques described above can be applied and adapted in variousways. For example, FIG. 13 illustrates a method for using the techniquesdiscussed above to validate a MRAC—that is, to determine whether aproposed MRAC will remain stable during operation. This method may beapplied, for example, by an engineer while designing the MRAC and/or bya technician tuning the system for a particular user/use. The method ofFIG. 13 may be fully or semi-automated by a computer system and resultsin a determination of whether the MRAC is acceptably stable or whetherthe MRAC should be adjusted or replaced in order to ensure stability.

The method begins by applying the operator model to the MRAC (step1301), for example, as described above in reference to FIG. 4. Theinteractions of the operator/human model and the MRAC are then evaluatedto determine whether the MRAC provides control-variable-independentstability (e.g., whether the MRAC enables the system operation to remainstable regardless of the time delay associated with a human operator'sreaction to observed conditions) (step 1303). In implementations wherethe MRAC is being evaluated for time-delay-independent stability, thecomputer system may be configured to verify the delay-independentstability of the MRAC using the techniques outlined above in Theorem 1and/or Theorem 2.

If the selected MRAC is confirmed to providecontrol-variable-independent stability for a selected control variable(e.g., time-delay-independent stability), then the MRAC is validated andthe MRAC is used to control the vehicle system as illustrated in theexample of FIG. 2 above (step 1305). However, if the MRAC does notprovide control-variable-independent stability, a determined range ofcontrol variables is identified that provides stability (for example,using the techniques associated with Theorem 3, above) (step 1307). Forexample, the techniques described above can be used to determine a rangeof time delays, a range of pilot model poles, a range of pilot modelzeros, and/or a range of pilot model gains that can be confirmed toprovide stability (e.g., according to the techniques associated withTheorem 3 and the “Illustrative Example” described above). If theexpected control variables for operation of the system (e.g., for thesystem itself, for a particular user, or for all users) fall within thedetermined range (step 1309), then the MRAC is validated as providingacceptable stability and is used to control the vehicle system asillustrated in the example of FIG. 2 above (step 1305). However, if theexpected control variables for operation are outside of the range ofvariables that has been determined to provide for stability, then theMRAC must be either adjusted or replaced (step 1311).

In some implementations, the method of FIG. 13 is applied as a loopwhile adjusting particular parameters of a MRAC in order to tune theMRAC for stable operation. In other implementations, the method of FIG.13 is applied to a plurality of different MRACs to determine which onesare acceptable (i.e., stable) and which ones are not.

The techniques and framework described above can also be adapted to begovern the operation of a vehicle using the controller 101. FIG. 14illustrates one example of a method utilizing the methods for evaluatingstability and determining stability limits in order to regulate theactuator commands provided by the control signals from the controller101 in response to particular user input commands. The human/operatormodel is again applied to the MRAC (step 1401) and thecontrol-variable-independent stability of the MRAC (e.g., thetime-delay-independent stability) is evaluated (step 1403). If the MRACis determined to be stable independent of the particular controlvariable in question (e.g., time-delay), then the MRAC is validated andready to use in controlling the vehicle (e.g., as illustrated in FIG.2). However, if the MRAC is determined to not be stable for all controlvariables, a range of control variables is determined at which stabilityis expected (step 1407). Based on this determined range of stablecontrol variables, the controller 101 (e.g., in the system of FIGS. 1and 2) limits the actuator commands to ensure that the controlvariable(s) remain with the determined range (step 1409).

For example, in reference to FIGS. 7 and 8 and the associated discussionabove, a MRAC has been validated as stable for particular pilot modelpoles indicative of levels of pilot aggressiveness. The controller 101may be configured to adjust/regulate the actuator commands to limit the“aggressiveness” of aerial maneuvers to ensure that the performance ofthe airplane remains within the acceptable/stable range of controlvariables.

Thus, the invention provides, among other things, systems and methodsfor validating and ensuring the stability of a control architecture.Various features and advantages of the invention are set forth in thefollowing claims.

What is claimed is:
 1. A method of implementing a model referenceadaptive control (MRAC) for a vehicle system, the method comprising:defining a first feedback-loop-based MRAC scheme, wherein the firstfeedback-loop based MRAC scheme is configured to receive a controlcommand, apply a reference model to determine a desired action for thevehicle system based on the control command, determine an actuatorcommand based on the control command, transmit the actuator command toat least one actuator of the vehicle system, monitor a sensor todetermine an actual action of the vehicle system in response toapplication of the actuator command by the at least one actuator,determine a system error based on a difference between the desiredaction determined by the reference model and the actual action, andadjust at least one adaptive parameter used to determine the actuatorcommand based on the determined system error; applying a first operatormodel to the first feedback-loop-based MRAC scheme, wherein the firstoperator model is configured to adjust the control command based atleast in part on the actual action of the vehicle system and a referenceaction for the vehicle system with a time-delay; determining a stabilitylimit of a first operating parameter of the first feedback-loop-basedMRAC scheme based on the application of the first operator model to thefirst feedback-loop-based MRAC scheme; and validating the firstfeedback-loop-based MRAC scheme in response to determining that expectedoperating conditions of the first operating parameter are within thedetermined stability limit of the first operating parameter.
 2. Themethod of claim 1, further comprising: receiving, by an electronicprocess, the control command from a user control; and controlling thevehicle system by an electronic processor configured to apply the firstfeedback-loop-based MRAC scheme to generate the actuator command inresponse to a control command received from a user control.
 3. Themethod of claim 2, wherein receiving the control command from a usercontrol includes receiving a control command from a steering wheel,wherein the control command is indicative of a rotational position ofthe steering wheel.
 4. The method of claim 2, wherein determining thestability limit of the first operating parameter includes determiningwhether the first feedback-loop-based MRAC scheme will cause the systemerror to approach zero regardless of variations in the first operatingparameter due to human operator-based manipulations of the user control.5. The method of claim 2, wherein controlling the vehicle system by theelectronic processor further includes: determining, by the electronicprocessor, the actuator command based on the control command receivedfrom the user control and a previous actuator command value to ensurethat the first operating parameter remains within the determinedstability limit of the first operating parameter.
 6. The method of claim1, further comprising: determining that the expected operatingconditions of the first operating parameters are not within thedetermined stability limit of the first operating parameter and, inresponse, adjusting at least one parameter of the firstfeedback-loop-based MRAC scheme.
 7. The method of claim 1, furthercomprising: determining that the expected operating conditions of thefirst operating parameters are not within the determined stability limitof the first operating parameter and, in response, defining a secondfeedback-loop-based MRAC scheme and applying the first operator model tothe second feedback-loop-based MRAC scheme.
 8. The method of claim 1,wherein the first operating parameter of the first feedback-loop-basedMRAC scheme includes a time-delay indicative of a period of time betweenan occurrence of the actual action and a corresponding corrective actionapplied by an operator to a user control.
 9. The method of claim 8,wherein determining the stability limit of the first operating parameterof the first feedback-loop-based MRAC scheme includes determiningwhether the feedback-loop-based MRAC scheme will ensure that operationof the vehicle system remains stable regardless of a value of thetime-delay parameter.
 10. The method of claim 7, wherein determining thestability limit of the first operating parameter of the firstfeedback-loop-based MRAC scheme includes determining range of time-delayvalues for which the first feedback-loop-based MRAC scheme will ensurethat operation of the vehicle system remains stable, and whereinvalidating the first feedback-loop-based MRAC scheme includesdetermining that a range of expected time-delay values for the operatoris within the determine range of time-delay values.
 11. The method ofclaim 1, wherein the vehicle system includes an airplane control systemand wherein the first feedback-loop-based MRAC scheme is configured toadjust the actuator to counteract an external force acting on theairplane and to maintain a desired path of travel.
 12. The method ofclaim 11, wherein the external force acting on the airplane includesturbulence.
 13. The method of claim 1, wherein the vehicle systemincludes an automobile system and wherein the first feedback-loop-basedMRAC scheme is configured to regulate operation of at least one selectedfrom a group consisting of an automobile steering system and anautomobile braking system.
 14. The method of claim 1, wherein applyingthe first operator model to the first feedback-loop-based MRAC schemeincludes determining a mathematical model representative of the firstoperator model and apply the mathematical model of the first operatormodel to a mathematical model representative of the firstfeedback-loop-based MRAC scheme to determine an overall mathematicalmodel representative of system operation under parallel control of botha human operator and the first feedback-loop-based MRAC scheme.